Combined, the plants constitute 15% of the battery storage installed globally last year.

Each component, comprised of 16,000 lithium-ion battery cells, is meant to suck up power from the grid by day and then feed it back in as demand surges, according to the New York Times. Capable of powering 15,000 homes, the system was conceived as part of a backup against projected energy shortages following a huge methane leak at a natural gas facility near Los Angeles.

For Tesla—which invested $5 billion in its so-called Giga-factory in the Nevada desert—the project has particular importance: viable and cost-effective battery storage is critical to wind and solar energy eventually kicking fossil fuels off the grid. And beyond a handful of grid-scale pilots, this is the first time such a system has gone online.

“It’s sort of hard to comprehend sometimes the speed all this is going at,” Tesla Chief Technology Officer J.B. Straubel told Bloomberg, Friday. “Our storage is growing as fast as we can humanly scale it.”

Until recently, battery storage has been a far more expensive means of meeting demand surges than natural gas “peaker” plants. However a rapid fall in lithium ion battery prices over the past two years—driven by the proliferation of electric cars—has made the technology far more viable.

For more on Tesla batteries, watch Fortune’s video:

Lithium-ion batteries reportedly cost about half as much today as they did in 2014.

“I had relatively limited expectations for the battery industry in advance of 2020,” Michael J. Picker, president of the California Public Utilities Commission told the New York Times.

Picker continued: “I thought that it would not really accelerate and begin to penetrate the electric grid or the transportation world for a while to come. Once again, technology is clearly moving faster than we can regulate.”